The invention relates generally to prefabricated housing and, more particularly, relates to underground tornado shelters.
Tornadoes are among the most devastating of natural phenomena. A single tornado can destroy houses and neighborhoods. Because tornadoes are difficult to predict, tornadoes can strike with little notice to the people threatened by them.
Scientists know that some geographic areas are prone to have tornadoes. Accordingly, people who live in those areas should take precautions to protect their lives in the event of a tornado. Specifically, someone who lives in an area prone to tornadoes can install an underground tornado shelter that enables the person to survive a tornado that destroys the person""s home.
Underground tornado shelters are known in the art. Typically, tornado shelters are constructed from concrete, steel, or fiberglass. Fiberglass tornado shelters have been popular in recent years because they offer many advantages over concrete or steel tornado shelters.
To make a conventional fiberglass tornado shelter, a manufacturer typically constructs a dome-shaped mold for the upper half of the tornado shelter and a hemispherical mold for the lower half of the tornado shelter. A worker then sprays layers of fiberglass material onto the molds. When the fiberglass hardens, the worker removes the upper half and the lower half from the molds. The upper half and the lower half of the tornado shelter each have a single wall of fiberglass forming the structure. The worker then attaches the upper half of the tornado shelter to the lower half of the tornado shelter using bolts. The manufacturer may then transport the tornado shelter to the customer.
To install the tornado shelter, workers excavate the earth to form a hole. They then place the tornado shelter into the hole. There are many different ways to anchor the tornado shelter in the hole. In one example, the workers insert pipes into cylindrical receptacles in the bottom of the lower half of the shelter. The pipes extend beyond the bottom of the lower half of the shelter. Once the workers have placed the shelter in the ground, they put concrete slabs over the portions of pipe extending beyond the bottom of the lower half of the shelter. These concrete slabs anchor the tornado shelter inside the ground. Other systems known in the art are also useful for anchoring the tornado shelter in the ground. The workers then bury the tornado shelter with dirt, leaving only an entrance to the tornado shelter exposed to the surface.
Although fiberglass tornado shelters are currently the industry standard, construction of tornado shelters from fiberglass has several disadvantages. For example, the molding process is slow, so production capacity is limited. Furthermore, molding a tornado shelter from fiberglass in the described manner requires a skilled operator. Because skilled operators command relatively high wages, the molding process using fiberglass is relatively expensive. Also, the thickness of the fiberglass walls greatly affects the cost and structural integrity of the shelter. Because the thickness of the fiberglass walls is difficult to control, the quality of the finished product varies according to the skillfulness of the operator. Furthermore, fiberglass is brittle, so tornado shelters constructed of fiberglass are sometimes damaged during transport to the installation location.
Due to the nature of the molding process using fiberglass, only the surface of the fiberglass wall that was in contact with the mold is smooth. The side of the wall opposite the side contacting the mold is rough, resulting in an unappealing finish.
Additionally, the nature of the molding process using fiberglass does not easily allow the creation of bolt holes on the upper half of the shelter that match corresponding bolt holes on the lower half. Thus, attaching the upper half of a tornado shelter made from fiberglass to the lower half is difficult. Furthermore, the completed tornado shelter is often not leak-proof because bolts are an imperfect method for joining the upper and lower halves together. Although a gasket material may be used to improve the seal between the upper half and the lower half, the gasket material may fail and is difficult to repair. As a result of the disadvantages inherent in a fiberglass tornado shelter built as described, there is a need in the art for an improved tornado shelter and an improved method for manufacturing a tornado shelter.
The present invention meets the needs described above in an improved tornado shelter constructed through a rotational molding process from any moldable material, such as high-density polypropylene or linear medium-density polyethylene. These materials are more durable and impact resistant than fiberglass. Accordingly, a tornado shelter constructed of such material is less likely to be damaged during transport to the installation location than a tornado shelter constructed of fiberglass.
By using rotational molding to construct the tornado shelter, numerous other advantages are realized. For example, the quality of the finished product is less dependent on the skill of the worker manufacturing the components of the shelter. Accordingly, the manufacturer of the shelter saves expenses because the manufacturer can more easily train workers to produce high quality tornado shelters. Furthermore, the manufacturer""s production capacity is increased because producing a tornado shelter through rotational molding is quicker than producing a tornado shelter using fiberglass.
According to one aspect of the invention, the rotational molding process is used to create an improved tornado shelter having a double-hull design. In other words, both the upper half and the lower half of the tornado shelter have an inner wall of linear medium-density polyethylene, an outer wall of polyethylene, and an optional layer of insulation between the inner and outer walls. Although linear medium-density polyethylene is used in a preferred embodiment, those skilled in the art will appreciate that other moldable materials such as polypropylene could also be used. This double-hull design offers numerous advantages over a conventional fiberglass tornado shelter having only a single layer of fiberglass. For example, the visible interior and exterior surfaces of the tornado shelter are smooth. This creates a more appealing and finished appearance than can be achieved with conventional single-hull fiberglass tornado shelters. Additionally, the layer of insulation provides structural support for the improved tornado shelter. Also, the insulation layer helps to prevent condensation from forming on the interior walls of the improved tornado shelter, which is a frequent problem with conventional single-hull fiberglass tornado shelters.
Through the rotational molding process, more exact design specifications can be met, allowing the manufacturer to realize additional advantages. For instance, the two halves of the improved tornado shelter can be manufactured with flanges around the periphery of their external surfaces. The flange on the upper half of the shelter is constructed to mate with the flange on the lower half of the shelter. This allows the person building the improved shelter to more easily align the halves and fasten them together. Also, due to the nature of the rotational molding process, the manufacturer can accurately control the amount of polyethylene or other material used to construct the shelter. Thus, the present invention enables the manufacturer of the improved tornado shelter to ensure that the thickness of the inner and outer walls of the tornado shelter match design specifications.
According to another aspect of the invention, the upper half of the tornado shelter is attached to the lower half by placing a bonding material (e.g., linear medium-density polyethylene or high-density polypropylene) embedded with wire between the flanges located on each of the halves of the shelter. An example of such a bonding material embedded with wire is POWERCORE. When electrical current is run through the wire, the bonding material melts. By allowing the bonding material to solidify while in contact with both flanges, a tight seal is created between the two halves of the improved tornado shelter. The bonding material embedded with wire may be applied in concentric rings. Applying the bonding material embedded with wire in concentric rings is also referred to as applying the material in multiple runs.
The tight seal created in this manner is generally superior to the seal created by connecting the two halves of a fiberglass tornado shelter using bolts. Furthermore, if a leak does occur in the improved tornado shelter, it is easier to fix. To fix such a leak, one simply melts the bonding material by again running current through the embedded wire and then allows the bonding material to solidify.
Generally described, the improved tornado shelter comprises a dome-shaped upper half having a first flange and a hemispherical lower half having a second flange with a shape mirroring the first flange. The upper half and the lower half each include an inner wall shaped through a rotational molding process and an outer wall shaped through the same rotational molding process. This combination of an inner and an outer wall is known as a xe2x80x9cdouble-hullxe2x80x9d design. Each half of the tornado shelter may also have a layer of insulation that is created during the rotational molding process and which is located between the inner wall and the outer wall.
The tornado shelter also has a means for forming a tight seal between the first flange and the second flange. For example, through-bolts or clamps could be used to hold the two flanges tightly together, and using a gasket either between the flanges or outside the flanges achieves a tight seal. Alternatively, the seal between the two flanges may be the result of welding the flanges together or using a chemical bonding agent between them.
Alternatively, as described above, a bonding material embedded with wire may also form the tight seal between the two flanges. An example of such a material is POWERCORE. The bonding material may be a plastic, such as highdensity polypropylene or linear medium-density polyethylene. To fuse the two halves of the shelter together, the bonding material embedded with wire is placed in solid form between the first flange and the second flange. When electrical current is run through the wire, the bonding material melts. Upon solidification, the bonding material bonds to both the first flange and the second flange, thereby fusing the two halves of the tornado shelter together.
The two halves of the tornado shelter may have additional features. For example, the upper half may be molded to define an opening through which a person can enter the tornado shelter. The lower half may have steps that enable someone to walk down into the shelter. The lower half may also have a circular seat on the floor inside the lower half. Both halves may be molded to include a pattern of ribs and indentations that reinforces the tornado shelter.
Additionally, the improved tornado shelter may have an anchoring system. Typically, the anchoring system is integral with the lower half of the shelter and is formed through the same rotational molding process used to shape the rest of the lower half.
In one embodiment, the anchoring system may have several pipe alignment grooves. A completed tornado shelter may also have pipes and plates used to help anchor the tornado shelter. The pipes lie in the pipe alignment grooves. Each plate connects edges of one of the pipe alignment grooves. For each plate, there is a means for fastening that plate to the anchoring system, thereby ensuring that the plates secure the pipes in their respective pipe alignment grooves. The pipes may each have two ends extending beyond the anchoring system.
Alternatively, the anchoring system may have several cylindrical spaces defined by the molding on the bottom of the tornado shelter. The pipes then lie in the cylindrical spaces such that the two ends of the pipes extend beyond the anchoring system.
In both anchoring systems, a slab of material (such as concrete) is placed on top of the portions of the pipes extending beyond the anchoring system. When the tornado shelter is buried, the surface areas of these slabs prevents the tornado shelter from rising in the ground. Because weight is not a crucial factor, the slabs may alternatively comprise the same material used to mold the rest of the tornado shelter or another moldable material. In fact, the slabs themselves may be molded using a rotational molding process. These slabs may have grooves that help the slabs to rest more securely on the pipes.
Through a means for aligning the upper half of the tornado shelter with the lower half, the tornado shelter may provide for easy assembly. Specifically, the means for aligning the two halves may include several boss and socket arrangements on opposing flanges. When the bosses slide into the socket receptacles, the two halves of the shelter are properly aligned. In another embodiment, the two halves of the shelter may each have corresponding sockets connected by pins.
Another means for aligning the upper half of the tornado shelter with the lower half involves placing bolts through corresponding holes on opposing flanges of the shelter. To increase the alignment range, two alternative types of holes are especially useful: 1) oversized holes that are larger than the bolts, or 2) inverted, conical holes, where the wide ends of the conical holes face each other. The conical-type holes (wide ends facing each other) can more easily accommodate the draft requirements of the molding processes.
In another embodiment, the present invention provides a process for making an underground tornado shelter. To make the tornado shelter, a manufacturer rotationally molds a dome-shaped upper half and a hemispherical lower half. The material used in the rotational molding process may be linear medium-density polyethylene or any other moldable material. The upper half includes a first flange and a first double-hull having a first inner wall, a first outer wall, and optionally a first layer of insulation between the first inner wall and the first outer wall. The lower half includes a second flange, a second double-hull, and an anchoring system having several pipe alignment grooves. The second double-hull has a second inner wall, a second outer wall, and optionally a second layer of insulation between the second inner wall and the second outer wall. After rotationally molding the two halves of the shelter, the manufacturer forms a tight seal between the first flange and the second flange.
In order to install the shelter, the manufacturer places pipes with two ends extending beyond the anchoring system into the pipe alignment grooves. Then, the manufacturer secures the pipes in the grooves by fastening to the anchoring system plates that connect edges of the pipe alignment grooves.
To finish installing the shelter, workers excavate the earth to define a hole in which they can bury the tornado shelter. After placing the tornado shelter in the hole defined by the excavated earth, they place slabs on the ends of the pipes to anchor the tornado shelter in the hole. They then bury the tornado shelter, perhaps with dirt, in a manner that exposes to the surface an entrance to the shelter.
The various aspects of the present invention may be more clearly understood and appreciated from a review of the following detailed description of the disclosed embodiments and by reference to the appended drawings and claims.